The present invention relates to a driving device for a stepping motor. More particularly, the present invention relates to a driving device for a stepping motor, which is used for a meter mounted on a vehicle, the initialization processing of which is improved.
Recently, stepping motors have been frequently used for speedometers to display a vehicle speed or for tachometers to display a rotating speed of an engine for the reasons of improving the indicating accuracy and reducing the manufacturing cost.
However, in a vehicle on which a meter having the above stepping motor is mounted, by an erroneous signal generated by vibration or noise of the vehicle, an actual moving distance of an indicating needle of the meter, which is linked with the rotation of the stepping motor, becomes different from an original moving distance of the indicating needle of the meter.
Therefore, in the case of the meter mounted on a vehicle in which this stepping motor is used, the following initialization processing is conducted. For example, at the time when the ignition switch has been turned on, the stepping motor is reversed in the direction of the stopper, and the indicating needle is returned to the zero position determined by the stopper.
In this initialization processing, in order to detect whether or not the indicating needle, the position of which is controlled by the stepping motor, comes into contact with the stopper by which the zero position of the indicating needle is determined, an induced voltage generated by the rotation of the rotor of the stepper motor is detected, and when the thus detected induced voltage becomes not more than a predetermined threshold value, the zero position detecting processing is conducted in which the indicating needle has collided with the stopper, which is set at the zero position, and stopped.
Referring to FIGS. 14 and 15, this zero position detecting processing is explained below.
FIG. 14 is a diagram showing a relation among each excitation step in the zero position detecting processing, the zero position detecting excitation pattern, the detection timing and the induced voltage. FIG. 15 is a view showing a relation between each excitation step in FIG. 14 and the rotating pattern of the rotor. Numerals in the parentheses represent the rotary angles of the rotor. Numerals in the rectangles represent the step numbers. In this connection, in these views, it is estimated that the rotor of the stepping motor is rotated in the direction indicated by an arrow at the time of zero position detection processing. This rotor has three N-poles and S-poles which are alternately arranged and uniformly magnetized.
An exciting signal for rotating the rotor is composed of exciting pulses P1, P2, P3 and P4 in which H (high level) and L (low level) are combined. For example, H is 5 volt and L is 0 volt. Exciting pulses P1 and P2 are supplied to both end portions “a” and “b” of one exciting coil 1a1. Exciting pulses P3 and P4 are supplied to both end portions “a” and “b” of the other exciting coil 1a2.
In order to reverse the rotor so that the indicating needle, which is connected to the rotor via a gear, can be moved to the zero position determined by the stopper, one cycle of the zero position detection exciting pattern is composed of eight exciting steps 3, 2, 1, 8, 7, 6, 5 and 4 to which the same period of time is allotted, and the stepping motor is driven by the half step drive system.
The exciting signals (P1, P2, P3 and P4) in the exciting step 1 are synchronized with the corresponding rotary patterns of the rotor. When the exciting step shifts in the order of 3, 2, 1, 8, 7, 6, 5 and 4, the rotor is rotated by 15° at a time as shown in FIG. 15. For example, when the step changes from the exciting step 3 to the exciting step 2, the angle of the rotor is changed from the rotary angle 0° to 15° by the exciting signal (P1, P2, P3 and P4). The rotary angle is changed between the exciting steps by 15° at a time in the same manner. In this connection, when the step shifts from the exciting step 4 to the exciting step 3 in the next cycle, the rotary angle is changed by 15°.
The cycle composed of the above eight exciting steps is repeated until the indicating needle comes into contact with the stopper, that is, until the rotor can not rotate and the induced voltage detected by the exciting coil becomes lower than the threshold value.
The detection timing signal for detecting the zero position is set so that it can become H at the timing when the exciting coil 1a1 is not excited, that is, at the exciting steps 1 and 5 in which the exciting pulses P1 and P2 supplied to both end portions “a” and “b” of the exciting coil 1a1 become L (zero volt) and at the exciting steps 3 and 7 in which the exciting pulses P3 and P4 supplied to both end portions “a” and “b” of the exciting coil C2 become L (zero volt). In response to this detection timing signal, the induced voltage, which is detected by the exciting coils 1a1, 1a2, one end of which is connected to the ground and the other end of which is open, is compared with the threshold value (reference voltage V). When the rotor is rotated so that the indicating needle can be moved to the zero position determined by the stopper, the indicating needle comes into contact with the stopper. Then, the induced voltage theoretically becomes zero. Therefore, the induced voltage becomes lower than the threshold value, that is, the zero position detection is conducted at this point of time.
In the above zero position detection processing, the following problems may be encountered. In the zero position detection processing, the detection cycle for detecting the induced voltage is used for the half step drive system. Therefore, as shown in the vector diagram of FIG. 16, an intensity of drive torque is greatly changed between one phase exciting step and two phase exciting step in the coil A-phase (the exciting coil 1a1) and the coil B-phase (the exciting coil 1a2). Therefore, the drive torque is not constant. Further, since the rotating speed of the rotor is changed, the rotor can not be rotated smoothly. Therefore, as shown in FIG. 17, the detected induced voltage-considerably changes. Accordingly, there is a possibility of the occurrence of erroneous detection of the zero point.